The present invention relates to a variable power lens system for use in a copying system such as a copying machine, in which the enlargement and the reduction can be carried out while maintaining the distance between the object and image surfaces constant.
In a conventional copying variable power lens system of such a type, the variable power ratio is small and the variable power range covers only from the equal magnification to the reduction magnification. Such a type of a lens system is, for instance, disclosed in the Japanese Laid Open Patent Application 44544/1979 and in the Japanese Patent Application 77925/1979 which is assigned to the present assignee. The Japanese Patent Application 77925/1979 exactly corresponds to U.S. patent application Ser. No. 154,261 filed on May 29, 1980 now U.S. Pat. No. 4,333,711 and entitled "Zoom Lens for Copying". However, the proposed lens system is composed of two lens groups and has a small variable power ratio less than 2. In addition, the first lens group has a negative focal length while the second lens group has a positive focal length. This type of lens system is so called a "retrofocus type". Accordingly, the distance between the object surface and the lens is short in comparison with the distance between the lens and the image surface. In case of the copying machine in which a scanning mirror is disposed between the object surface and the lens, it is therefore disadvantageous that a sufficient space therefor cannot be formed.